1. Field of the Invention
The present invention relates to an objective lens disposed at a tip portion of an endoscope and, in particular, to an objective lens for an endoscope, in which a ray direction changing prism is placed on the image surface side thereof.
2. Description of the Prior Art
In a direct-vision type endoscope employing a solid-state imaging device, the latter is often axially inserted and disposed at the tip portion of the endoscope. In such an endoscope, a ray direction changing prism is inserted and disposed between its objective lens and solid-state imaging device in general.
Since the size of the ray direction changing prism is determined by the image size, it is necessary to fully secure the distance between the last surface of the objective lens and the imaging position between which the ray direction changing prism is inserted and disposed, i.e., back focus.
While the image size is reduced as the solid-state imaging device is made smaller, flare and ghost may occur unless a sufficient margin is provided for the distance between the prism wall surface and the effective luminous flux. Accordingly, in view of the processing accuracy of parts and their assembling accuracy, the above-mentioned distance cannot be shortened extremely, thus making it difficult to reduce the prism size in proportion to the image size. Consequently, an objective lens having a longer back focus as compared with its focal length is necessary.
However, in response to demands for wider angle, the objective lens for an endoscope tends to have shorter focal length even at the same image size, thus making it difficult to obtain sufficient back focus.
On the other hand, Japanese Patent Application No. 6-251976 Japanese Unexamined Patent Publication No. 8-122632) proposes an objective lens system, whose front-group concave lens system is composed of a single concave lens and a single convex lens, in which chromatic aberration in magnification is favorably corrected. This objective lens system, however, may be insufficient in terms of back focus.